1. Field of the Invention
The present invention relates to optical angle measuring instruments. More particularly, this invention pertains to an improved sextant.
2. Description of the Prior Art
The rough setting of the alhidade along the arc of the limbus toothing requires disengagement of the drum screw (connected to the measuring drum) from the limbus. In the prior art, the drum screw bearing articulated at the lower free end of the alhidade is disengaged by a spring-biased catch and pivoted so that the drum screw is disengaged from the limbus toothing. The disengaging member and the catch are relatively far removed from the limbus toothing and the engagement region of the drum screw. As a consequence, a bending moment can act on the alhidade if the actuation force acting on the catch does not act in at least an approximately tangential direction with respect to the limbus arc. As a result, indirectly caused measuring errors may occur. Additional measuring errors or inaccuracies can arise as a result of the fact that the point of engagement of the (normally conical) drum screw is slightly displaced by unavoidable production tolerances of the alhidade toothing. The moment of contact pressure, which acts, for example, through a bent leaf spring in the measuring drum bearing onto the end of the drum screw remote from the region of engagement may change when the catch is (unknowingly) slightly pressed by the user. A 0.1 mm diameter offset in the radial direction in the region of engagement of the conical drum screw can alter the lead angle by about 50" (sec).
A further problem that occurs in practice which gradually worsens the measuring quality of angle-measuring instruments of the above-mentioned type arises from the situation in which the catch by which the drum screw is pivoted to come free from the limbus toothing is not completely pressed. In such a case the user begins the rough adjustment by pivoting the alhidade before the drum screw is completely out of engagement with the limbus toothing causing the drum screw to "chatter" along the limbus toothing. As a result, the measuring accuracy of the instrument worsens and such inaccuracy inevitably has to be accepted over time even when hardened materials are used for the drum screw and the limbus toothing. Taking into account the usual geometry of the graduated arc or limbus, an alteration resulting from improper handling of only 1/1000 mm will degrade measuring accuracy by 2.5 arc seconds.